Corrosion Tips Archive Page 2

Below is an archive of all previous corrosion tips organised in chronological order.

Tip 11 - Choosing a Corrosion Consultant

The field of corrosion is vast, there’s a lot more too it than rusty old iron! Different alloys, whether mild steel, copper alloys, stainless steels, aluminium alloys, titanium etc. interact with a whole variety of environments in very different ways. Corrosion in the many different forms is in turn influenced by a whole range of factors, including temperature, flow rates, solution concentration, pH, degree of aeration and presence of particulate matter. There are also many different ways to try to prevent or mitigate against corrosion, including paints and coatings, inhibitors and cathodic protection.

Not surprisingly, there is no corrosion consultant, who is an expert in all these fields. It takes many years (often decades) of experience to properly understand real corrosion failures. In order to properly carry out corrosion investigations, one also needs to have a good appreciation of metallurgy, water chemistry and the engineering aspects of system design and operation. Usually, straight metallurgists, chemists or engineers will look at the problem from only one angle and fail to understand the whole picture. The result, all too often is that wrong conclusions are drawn and wrong steps are then taken to reduce the risk of the problem occurring in future.

With a good solid background in corrosion, even if he hasn’t come across the particular failure before, the corrosion consultant will be able to apply his knowledge or know where to find the right information. This is why choosing a good corrosion consultant, who has proven knowledge and experience of the particular area of concern, is very important.

Tip 10 - Avoidance of Corrosion in Re-circulating Heating and Cooling Systems

The main differences between these systems and the domestic water systems discussed in tip 9 in the factors influencing corrosion are the level of aeration of the water and the water composition. In domestic water systems, the water is essentially saturated or virtually saturated with dissolved oxygen, which means that unalloyed steels cannot be used since they would corrode very quickly. Also, in contrast to drinking water systems, the chemical composition of water in re-circulating cooling and heating systems can differ greatly from that of the fill water and in addition, much more variety of corrosion and scale inhibitors can be added.

In order to minimise the corrosion risk, the following advice should be followed:

  1. Make sure the system is airtight by correctly sizing and locating any expansion vessels and checking that the gas pressure is correctly set. Undersized or incorrectly set gas pressures will mean that expansion of water on heating is not properly accommodated for and water is then lost through the pressure relief valve. Fresh aerated water is then added to the system to maintain pressure.
  2. If plastic pipe is being used, use only barrier plastic pipe to prevent diffusion of air into the system.
  3. For open-vented systems, check that the pump is correctly located and sized so that pumping over of water into the header tank does not occur.
  4. Chemically clean systems after installation according to BS 9503 and inhibit with a recognised make of corrosion inhibitor (if possible use one which has passed the new Build-Cert approved DWTA inhibitor evaluation test). Chemical cleaning will remove flux residues and installation debris from the system. Inhibitor manufacturers should supply test kits to check inhibitor levels.
  5. When in service, periodically check that air is not being drawn into the system by opening air vents in the top radiators. In addition, check that the system pressure is correct and adjust if necessary.
  6. If system pressures continually drop or air is found to accumulate in radiators, then the source of the problem must be established and corrected.

Further advice is to be found in BS 14868:2005.

Tip 9 - Avoidance of Corrosion in Domestic Water Systems

Enough of the theory for now – here are some practical tips for avoiding, or at least reducing the risk of, corrosion in drinking water systems (known as domestic water systems).

  1. Do not allow water to stagnate in the system for a long period after installation. Regular renewal with fresh aerated water is required to form protective patinas or corrosion product layers on copper and other metals in the system.
  2. Do not drain down the system after pressure testing. It will be impossible to remove all the water and localised attack will quickly develop at the three phase boundary: metal/water/air.
  3. Reduce as much as possible dead-legs in the system. These can lead to stagnant water conditions in use, which favours the development of pitting corrosion.
  4. Only use copper pipe to EN 1057 with the BS kitemark. Copper pipe not complying to this standard may still have carbon film residues on the inside from the extrusion process, which can lead to type I pitting in hard water areas.
  5. Do not used galvanised pipe in hot water systems and never use downstream of copper.
  6. Design a system so that water flows in copper pipework are in the range 0.5-1m/s in hot water and 0.5-2m/s in cold water. Too low water flows allow debris to settle out and may lead to under-deposit corrosion. Flow rates above these values may induce erosion corrosion on copper.
  7. For larger systems, if there is a risk of deposits being brought into the system, i.e. from surface upland waters, fit a water filter to the incoming mains.
  8. Deburr the cut ends of copper pipe and do not have large changes in section at joints. This will reduce the risk of turbulent flows immediately downstream of the joint and hence the likelihood of erosion corrosion.
  9. Do not use excessive soldering fluxes and wipe any excess from the outside of the pipe after soldering. Flux residues left after soldering may lead to pinholing of pipework several months later.
  10. Lag pipework, especially hot water pipes, not only to save energy but also to keep water temperatures in the pipes outside the range 25-50°C for any length of time. At these temperatures, micro-organisms can flourish which increases the risk of legionella and also later of microbial induced pitting corrosion.
  11. For systems with an open header tank, always make sure that the lid is securely fitted. This will prevent insects and even birds ending up in the tank stimulating microbial growth.

Tip 8 - Crevice Corrosion

Have you ever wondered why your old car always rusted in the wheel arches or under the sills first? Well, this is because of crevice corrosion.

Crevice corrosion, like pitting corrosion described in Tip 6, is a form of localised attack. It occurs due to a differential aeration effect, i.e. due to a lower dissolved oxygen content in the water within a crevice or under a deposit compared to the dissolved oxygen content in the bulk environment. This makes the metal within the crevice anodic compared to the rest of the metal surface and hence it corrodes preferentially. Usually, this takes the form of pitting attack within the crevice.

Because a crevice can be seen as a pre-existing pit, crevice corrosion occurs more readily than pitting on a bare, flat surface in the same environment. For this reason, crevice corrosion on stainless steels or aluminium occurs at a lower chloride content than is required for pitting corrosion. It is always a good idea to try to design out crevices for any metallic system, which will come into contact with water. If this is not possible, then more attention should be paid to the environmental conditions likely to be experienced in service.

Tip 7 - Corrosion Kinetics II

In corrosion tip 5 we showed how the corrosion rate of most metals is governed mainly by the protectiveness or otherwise of oxides or films, which form on the surface of the metal in the environment.

In acidic solutions, hydrogen evolution is the dominant cathodic reaction, which drives metal dissolution, i.e.

2H+ + 2e- → H2

And hence, the lower the pH the faster is this reaction, which is thermodynamically controlled. However, the other principle cathodic reaction, oxygen evolution is diffusion controlled. This is because dissolved oxygen in the water has to diffuse to the metal surface in order to be reduced to hydroxyl ions:

2H2O + O2 + 4e- → 4OH-

In many situations, e.g. for mild steel rusting in near neutral water, the oxygen gets consumed faster than it can diffuse through the water to the metal surface. Hence, the corrosion rate reaches a limit governed by the rate of diffusion of oxygen. The diffusion of oxygen to the metal surface is increased in flowing water so that as flow rate increases so does the corrosion rate. In an oxygen free acidic environment, however, the corrosion rate of mild steel or iron would be unaffected by the flow rate, as there would always be plenty of hydrogen ions at the metal surface, even in stagnant conditions, to be reduced.

Tip 6 - Introduction to Pitting Corrosion

Pitting corrosion is a form of localised corrosion, which can occur on most metals in specific environments. It can cause leakage or rupture of pipe-work or vessels and may leak to the development of stress corrosion cracks.

Pitting takes place in two stages; pit initiation and pit propagation. During the pit initiation stage, very little is visible to the naked eye, although localised breakdown of protective surface films takes place at anodic sites. This is often temperature critical, with pit initiation not occurring below a certain temperature. During the pit propagation or growth phase, local cell conditions become established as chloride and other anions are drawn into the pit site and the solution within the pit becomes acidic. The metal area surrounding the pit is cathodic to the pit and corrosion here is often negligible.

Growing pits may become stifled by corrosion products and may become inactive or inactive pits may become active again as conditions change. Pits usually grow faster on upward facing surfaces such as the 6 O’clock position in horizontal pipes, as in this situation the aggressive localised solution remains easier within the pit.

Aluminium or stainless steels are susceptible to pitting in chloride-containing waters, as the small Cl- ion is able to penetrate and break down the passive oxide film. However, the concentration of chloride needed to cause pitting depends very much on the alloy type and composition. For instance, super-duplex stainless steels will resist pitting in sea-water, while 304 austenitic stainless steels may pit in waters containing < 100mg/L Cl-. Pitting of low alloyed steels and iron occurs in oxygen rich waters under corrosion tubercles. Copper can undergo pitting in different water compositions depending on temperature (more about this later).

Tip 5 - Corrosion Kinetics I

In tips 2 and 3 we looked briefly at how corrosion is an electrochemical process, in which thermodynamics says what reactions are possible on an electrode (metal) surface and which metals corrode preferentially to others when coupled together. However, thermodynamics says nothing (or very little) about the rate of the corrosion process. For this, we have to look to the kinetics of the reactions.

In the real world, the rate of corrosion of metals and alloys in an environment is governed above all by the presence of surface layers and their protectiveness (This applies to high temperature oxidation, as much as to aqueous corrosion). Some alloys form a naturally protective, microscopically thin surface oxide on contact with air. These, so-called passive layers, are formed on stainless steels and aluminium alloys. Other metals, in the right conditions in water, can form passive oxide layers, which slow down the corrosion rate by several orders of magnitude. A good example of this is mild steel in low oxygen-containing water, which forms a passive layer of Fe3O4 (magnetite) on the surface

Although some metals and alloys do not form these passive layers, the corrosion rate in water is still governed by the formation of thicker macroscopic layers. The corrosion rate of galvanised coatings on steel is controlled by zinc oxide/hydroxide corrosion product layers. The corrosion of copper pipework in drinking water systems is greatly reduced by the formation of copper oxide and copper carbonate patinas.

It is when these protective layers either do not form properly in the first place or become disrupted for some reason that problems occur.

Tip 4 - Forms of Corrosion

Corrosion can take many different forms. These can be classified as: general corrosion, pitting corrosion; erosion corrosion; galvanic corrosion; selective corrosion; stress corrosion cracking and fatigue corrosion. Apart from general corrosion, all the others are types of localised corrosion.

A good example of general corrosion is rusting of steel. This is often unsightly but generally is a slow process and can be predicted and taken into account during design. Localised corrosion, on the other hand, is usually unpredictable and often occurs in a sudden and sometimes catastrophic manner.

Pitting corrosion can occur with all common construction metals. It is often responsible for pinholing of water pipes made out of copper, mild steel or even stainless steels. There are also many types of pitting corrosion – some can cause pinholing in a matter of months, while other types take many years for this to happen.

Erosion corrosion is caused principally by too high water flow rates resulting in turbulent flow conditions down a pipe. It can also with most materials, although the critical flow rates required vary greatly from metal to metal. For instance, erosion corrosion can occur in copper at flow rates as low as 1m/s but does not occur in stainless steels until a flow rate of perhaps 20m/s.

We touched on galvanic corrosion last week. Selective corrosion is a form of galvanic corrosion within an alloy, whereby the more active phase is preferentially dissolved leaving a porous matrix. The most common forms are dezincification of brasses and graphitisation of grey cast irons.

Stress corrosion cracking and corrosion fatigue require the combined effects of a tensile stress, a specific corrosive species or medium and a sensitive alloy. The stresses can be applied stresses or residual stresses from the manufacturing process. Typical examples of SCC are austenitic stainless steels in chloride-containing waters and alpha-beta brasses in ammonia-containing waters. Corrosion fatigue is similar to SCC, except that the applied stresses are cyclic, which often speeds up the process.

Tip 3 - Galvanic Series

If we immerse a metal in an aqueous environment e.g. seawater, anodic and cathodic reactions, as described in the previous tip, occur. Because these are electrochemical in nature, they take place at a certain voltage or potential, as measured against a standard reference electrode. Active metals such as zinc and aluminium have a high negative potential, whilst noble metals such as copper and stainless steel have less negative or even positive potential. The order of the metals and alloys in a particular environment forms what is termed a galvanic series, as shown below.

If two different metals are joined together in the particular environment, then corrosion of the more active metal is increased while corrosion of the less active or more noble metal is decreased. It is therefore not a good idea to couple aluminium alloys or mild steel to copper, since the corrosion rate of the steel and especially the aluminium will be increased. On the other hand, metals can be protected by coupling with more active metals, which corrode sacrificially. An example of this is ships hulls, which are protected by strapping zinc or magnesium anodes below the waterline.

In reality, if the potential difference between two metals or alloys is less than ~200mV, then the galvanic effect will be slight. Note also that the galvanic series cannot be used to predict corrosion rates.

Galvanic Series in Stagnant Sea Water:

Tip 2

Corrosion of metals can be split into 2 main types; high temperature corrosion and aqueous corrosion. High temperature corrosion occurs due to oxidisation of a metal or alloy and high temperatures usually above 350C. Aqueous corrosion is an electrochemical process, which requires (as the name suggests) liquid water to be present.

What we mean by electrochemical is that it involves both an oxidation process or anodic process, i.e. metal dissolution and a reduction or cathodic process, such as oxygen reduction or hydrogen evolution. In neutral waters, the main cathodic reaction is oxygen reduction, whilst in acids the dominant cathodic reaction is hydrogen evolution. Since in freely corroding situations, the anodic reaction rate must equal the cathodic reaction rate, a simple way of reducing the corrosion rate in neutral waters is to remove the dissolved oxygen.

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